In the past, Doppler radars have been used to measure aircraft velocity relative to the ground plane in order to bound the errors in the INS, however, the Radio Frequency, RF, emissions inherent to this technique increase the vulnerability of the aircraft to hostile threats. The errors in the INS solution may be calibrated and removed by the use of the navigation solution provided by a Global Position System (GPS) receiver. However, GPS accuracy can be degraded at the whim of the US Government and hence, this option for achieving a high accuracy navigation solution may not always be available. Finally, research and development effort is currently directed at systems that will compare sensory imagery of the outside world with prior knowledge of its state, in order to maintain an accurate estimate of the aircraft's position. Encouraging results from this work have been presented, but, to be viable operationally, large quantities of data will have to be gathered and processed at great expense, in order to generate the data map of the entire world with which the sensor imagery will have to be compared.
The present invention seeks to provide an INS positional accuracy enhancing system which is more covert than a Doppler radar, works independently of manmade external references and does not require the creation of large, expensive databases with which to compare sensor imagery.
The present invention also seeks to provide such a system which will work in areas of the world where the ground is flat and featureless, such as over oceans and deserts, where other self-contained systems, for instance, Terrain Reference Navigation are unable to operate.